The purpose of the research program is to define the nature of the defect in immune competence in aged humans. Immune function of human B-lymphocytes and human T-lymphocytes will be measured in vitro. Specifically the response of T-lymphocytes to plant lectins will be investigated in order to define the number of mitogen responsive cells and the proliferative capacity of their progeny. To evaluate the function of human B-lymphocytes the proliferative response of B-lymphocytes from young and old humans to staph protein A, which is a B-lymphocyte mitogen, will be compared. We will also evaluate the primary immune plaue forming cell response to sheep erthrocytes of lymphocytes from young and old humans.